In the field of the magnetic disc, a 2 MB MF-2HD flexible disc using Co-modified iron oxide has been generally loaded in a personal computer. However, along with the increase in the amount of data to be dealt with, the capacity thereof has become insufficient and the increase of the capacity of the flexible disc has been demanded.
In the field of the magnetic tape also, along with the prevalence of the office computer, such as minicomputers, personal computers and work stations, magnetic tapes for recording computer data as external storage media (a so-called backup tape) have been eagerly studied. For putting magnetic tapes for such usage to practical use, the enhancement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing performance (i.e., the increase of throughput) for achieving high capacity recording and the miniaturization.
Magnetic recording media comprising a nonmagnetic support having coated thereon a magnetic layer comprising an iron oxide, a Co-modified iron oxide, CrO2, a ferromagnetic metal powder (MP), or a hexagonal ferrite powder dispersed in a binder have been conventionally widely used. Of these powders, ferromagnetic metal fine powders and hexagonal ferrite fine powders are known to be excellent in high density recording characteristics. In the case of a disc, as high capacity discs using ferromagnetic metal fine powders which are excellent in high density recording characteristics, there are 10 MB MF-2TD and 21 MB MF-2SD, and as high capacity discs using hexagonal ferrite, there are 4 MB MF-2ED and 21 MB Floptical, however, any of these are not satisfactory with respect to capacities and performances. As is the circumstance, various attempts have been made to improve high density recording characteristics. For example, high capacity and high density recording of from 100 M to 200 M, e.g., LS-120 and ZIP, has been realized, and now even higher density recording of a real recording density of 0.2 G bit/inch2 or more has been demanded. In addition, due to the requirement for shortening access time, the rotation frequency of discs has a tendency to increase.
In such a high density, high rotational and high transfer magnetic recording medium, higher running property and higher durability than running property and durability of conventional media are required for maintaining stable recording and reproduction. Prior art techniques which are proposed for mainly improving recording density and running durability in coating type magnetic recording media are described below.
For example, Japanese Patent (Application) Laid-Open No. 52541/1994 discloses a magnetic tape having an average protrusion height of abrasives on the magnetic layer surface of 15 nm or less, and electromagnetic characteristics thereof are compatible with durability by improving head abrasion and head contamination. Japanese Patent Laid-Open No. 12651/1994 discloses a magnetic disc in which the magnetic layer has Ra of 15 nm or less, protrusions having a height of 30 nm or more are distributed from 125,000 to 250,000 per mm2 of the magnetic layer, and the amount of a lubricant in the magnetic layer is prescribed.
Japanese Patent Laid-Open No. 309650/1994 discloses a magnetic recording medium, in particular a magnetic disc, in which a magnetic layer contains from 8 to 30 mass parts (i.e., weight parts) of a lubricant based on 100 mass parts (i.e., weight parts) of the magnetic powder, and the number of protrusions having the height higher than the surface which is 20 nm lower than the highest protrusion is specified to be from 400 to 2,500 per mm2, that is, the amount of the lubricant in the magnetic layer and the existing density of protrusions having specific height on the magnetic layer surface are specified to thereby secure the running stability.
Magnetic heads making electromagnetic induction as a principle of operation (an induction type magnetic head) have been conventionally used and prevailed. However, magnetic heads have reached the limits of their abilities to be used for further higher density recording and reproduction. That is, it is necessary to increase the number of turns of a reproduction head coil to obtain higher reproduction output, but when the turn number of a reproduction head coil is increased, the inductance increases and the resistance at a high frequency region increases, as a result, the reproduction output decreases.
A reproducing head making MR (magneto resistance) as a principle of operation has been proposed in recent years and is coming to be used in a hard disc, and Japanese Patent Laid-Open No. 227517/1996 proposes the application of the head to a magnetic tape. The MR head can provide reproduction output of several times as large as that by the induction type magnetic head, and since no induction coil is used, equipment noise such as impedance noise is widely reduced, therefore, it has been possible to obtain a large S/N ratio by reducing the noise of a magnetic recording medium. In other words, good recording and reproduction can be done and high density recording characteristics can be drastically improved by lessening the noise of magnetic recording media hiding behind the instruments.
However, there is a problem with the MR head such that noise is generated by the influence of a small amount of heat (thermal noise), in particular, if the MR head is touched to the protrusions on the surface of a magnetic layer, noise is suddenly and continuously increased, and in the case of digital recording, the problem is sometimes worsened in such a degree as error correction is impossible. This problem of thermal noise is serious in a magnetic recording medium for use in a system of reproducing recorded signals of 0.5 G bit/inch2 or more of recording density.
It is important to control the surface property of a magnetic layer for reducing thermal noise, and controlling the surface property of a support has been done as a means for that purpose. For example, Japanese Patent Nos. 2938548 and 2938549 suggest supports provided with a polyurethane covering layer, and Japanese Patent Publication No. 57647/1993 suggests a support provided with a layer comprising a compound polymerizable by radiation irradiation.
However, the support produced by the former technique is expensive and insufficient in smoothness, and the latter technique requires an expensive irradiation apparatus and in some cases the support is damaged at covering.
Further, a smoothed support is supposed to be once rolled up in a roll according to both techniques and the wound roll is delivered again in a coating or deposition process separately provided for providing a functional layer such as a magnetic layer. However, a smoothed support is hard to deal with and is easily wrinkled at rolling-up operation, therefore, productivity and yield are remarkably low.